Spiral bevel gears are generally cut from a gear blank by a method known as face mill generation using a cutter with blades positioned on the circumference of a circle. The blades of a particular cutter may be selected for roughing, finishing, completing and other such steps. As the cutter rotates, the blades remove material from the blank, as may be observed in FIG. 1 in which a conventional spiral bevel gear cutting arrangement is illustrated. A similar action can be obtained with a cup-shaped grind stone.
While the cutter rotates, it orbits about a machine axis and the blank rotates to simulate a gear mesh. Each descrete orbital motion of the rotating cutter in synchronization with the blank, known as a "roll," generates a single tooth surface or two facing tooth surfaces having the desired lengthwise curvature. Although the curvature is arcuate, it approximates the theoretically optimum spiral configuration.
This gear cutting technique has been used for many years and constitutes the basic principal of operation of a wide variety of gear cutting machines made by Gleason Works of Rochester, N.Y.
When designing a spiral bevel gear to be cut in the manner described above, a large number of interrelated variables must be considered. Once the basic size and pitch cone of the gear have been selected, the cutter diameter and the cutter distance (the radius on which the cutter orbits about a machine axis) are of major importance. These variables determine, to a large extent, the mean spiral angle. The mean spiral angle is the angle between a pitch cone element (PCE) and a tangent to the lengthwise curvature of the tooth at the mean cone distance. Conventionally, spiral angles vary from about +35 to zero degrees, a gear with a zero spiral angle sometime being known as a Zerol gear. A mean spiral angle of less than +15 degrees is unusual in highly loaded environments.
A conventional spiral bevel gear, shown in FIG. 2, has teeth that are oriented in such a way that the concave tooth surfaces face inwardly toward the center of the representative gear 10', while the convex tooth surfaces face outwardly away from the gear center. In FIG. 2, the cutter periphery is represented by the line CP'. It will be noted that this line intrudes into the area at the center of the gear to be occupied by a shaft or other central structure, and in this typical configuration reaches almost to the center of the gear 10'. This intrusion can be reduced by decreasing the spiral angle and the cutter radius. It is therefore possible to cut a spiral bevel gear in such a way that a relatively thin shaft made integral with the gear blank does not interfere with the cutter. More typically, however, a gear shaft protruding from the toothed face of the gear occupies at least the major portion of the central area within the gear and cannot be cleared by the cutter. It is therefore necessary to form the gear and the shaft separately and use a gear-shaft joint to connect them.
Joints are disadvantageous because they increase weight and cost while reducing accuracy, load capacity and reliability. The reduction in reliability, perhaps the most important disadvantage, is caused by fretting which leads to fatigue failure. Fretting can be severe and is difficult or impossible to detect because the debris that it generates becomes trapped within the joint. Efforts to avoid the effects of fretting lead to increased manufacturing costs and the use of massive joints.
The problems associated with gear-shaft joints are particularly acute when the gear is subjected to high loads as in some aircraft applications, particularly in helicopters. Failure of a shaft or joint in this environment could be catastrophic, resulting in a loss of aircraft, crew and cargo. A shaft that would be cleared by the cutter may be to weak to be acceptable and may not be usable for straddle mounting of the gear. Powdered metallurgy and gear casting techniques that would permit the formation of an integral gear and large shaft cannot, at the present state of those technologies, produce a structure capable of withstanding high loads.
There is, therefore, a need for a gear cut from a blank that can be made integral with a large diameter shaft and is capable of withstanding high loads.